Generally, the process of electrophotographic printing includes charging a photoconductive member to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the photoconductive surface is exposed to a light image from either a scanning laser bean or an original document being reproduced. This records an electrostatic latent image on the photoconductive surface. After the electrostatic latent image is recorded on the photoconductive surface, the latent image is developed. Two component and single component developer materials are commonly used for development. A typical two component developer comprises magnetic carrier granules having toner particles adhering triboelectrically thereto. A single component developer material typically comprises toner particles. Toner particles are attracted to the latent image forming a toner powder image on the photoconductive surface, the toner powder image is subsequently transferred to a copy sheet, and finally, the toner powder image is heated to permanently fuse it to the copy sheet in image configuration.
The electrophotographic marking process given above can be modified to produce color images. One color electrophotographic marking process, called image on image processing, superimposes toner powder images of different color toners onto the photoreceptor prior to the transfer of the composite toner powder image onto the substrate. While image on image process is beneficial, it has several problems. For example, when recharging the photoreceptor in preparation for creating another color toner powder image it is important to level the voltages between the previously toned and the untoned areas of the photoreceptor.
In the application of the toner to the latent electrostatic images contained on the charge-retentive surface, it is necessary to transport the toner from a developer housing to the surface. A basic limitation of conventional xerographic development systems, including both magnetic brush and single component, is the inability to deliver toner (i.e. charged pigment) to the latent images without creating large adhesive forces between the toner and the conveyor, which transport the toner to latent images. As will be appreciated, large fluctuation (i.e. noise) in the adhesive forces that cause the pigment to tenaciously adhere to the carrier severely limit the sensitivity of the developer system thereby necessitating higher contrast voltages forming the images. Accordingly, it is desirable to reduce such noise particularly in connection with latent images formed by contrasting voltages.
In order to minimize the creation of such fluctuation in adhesive forces, there is provided, in the preferred embodiment of the invention a toner conveyor including means for generating traveling electrostatic waves which can move the toner about the surface of the conveyor with minimal contact therewith.
Traveling waves have been employed for transporting toner particles in a development system, for example U.S. Pat. No. 4,647,179 to Schmidlin, which is hereby incorporated by reference. In that patent, the traveling wave is generated by alternating voltages of three or more phases applied to a linear array of conductors placed abut the outer periphery of the conveyor. The force F for moving the toner about the conveyor is equal QE t where Q is the charge on the toner and E t is the tangential field supplied by a multi-phase AC voltage applied to the array of conductors.
In that Patent, toner is presented to the conveyor by means of a magnetic brush, which is rotated in the same direction as the traveling wave. This gives an initial velocity to the toner particles, which enables toner having a much lower charge to be propelled by the wave. Typical approaches in the past have used a magnetic brush to load toner to the traveling wave grid. These approaches will mechanically wear the traveling wave device at the loading zone (grinding at a stationary loading zone on the grid). These approaches are also limited in the amount of toner they expose to stripping because the magnetic brush tips tend to be sparse for large brush spacing and the stripping field on the traveling wave grid decreases exponentially with distance from the grid surface. The methods to increase the amount of toner loaded to the grid (with the magnetic brush in this mode) include speeding up the magnetic roll, decreasing the spacing, increasing the loading zone length, and increasing the number of rolls. These methods all will result in increased wear on the grid.
Fluidized beds have been used to provide a means for storing, mixing and transporting toner in certain single component development systems and loading onto developer rolls. Efficient means for fluidizing toner and charging the particles within the fluidized bed are disclosed in U.S. Pat. No. 4,777,106 and U.S. Pat. No. 5,532,100, which are hereby incorporated by reference. In these disclosures, corona devices are embedded in the fluidized toner for simultaneous toner charging and deposition onto a receiver roll. While the development system as described has been found satisfactory in some development applications, it leaves something to be desired in the way in applications requiring the blending of two or more dry powder toners to achieve custom color development. Also, it has been found in the above systems that there are frequently disturbances to the flow in the fluidized bed associated with charged particles in the high electric fields surrounding corona devices immersed in the reservoir. Also, wire contamination present a reliability issue.
Triboelectric charging (contact electrification) of dry toners is a standard method used to electrically charge toner particles for development of latent electrostatic images. An alternate method to charge toners is via ion bombardment (Ion Charging) which offers many advantages, especially in applications to custom color where "in-situ" toner mixing is advantageous.
Triboelectric charging of colored toners requires different additives dependent on toner color to achieve stable charging whereas ion charging of toners offers the advantage of charging toner particles based mainly on their size, independent of their intrinsic composition and surface structure. Triboelectric charging of toners also can create localized patches of charge on the toner particles which can lead to strong adhesion of these toners to various surfaces requiring special measures to remove them in the development, transfer and cleaning steps in the xerographic process. In the ion charging process, charged ions bombarding the toner particles are driven by the net field around the particles which tends to uniformly charge the toner, helping to decrease adhesion of these toners to donor or photoreceptor surfaces. One method to charge toner via ion bombardment involves fluidizing the toner and charging it using corona generation in close proximity to this fluidized bed.
Typical approaches in the past have used a magnetic brush to load toner to the traveling wave grid. These approaches will mechanically wear the traveling wave device at the loading zone (grinding at a stationary loading zone on the grid).
These approaches are also limited in the amount of toner they expose to stripping because the magnetic brush tips tend to be sparse for large brush spacing and the stripping field on the traveling wave grid decreases exponentially with distance from the grid surface. The methods to increase the amount of toner loaded to the grid (with the magnetic brush in this mode) include speeding up the magnetic roll, decreasing the spacing, increasing the loading zone length, and increasing the number of rolls. These methods all will result in increased wear on the grid.
At the development zone there are a number of issues which need to be addressed. When toner is presented to a latent electrostatic image in the development zone it is necessary to control the toner cloud height and speed at the entrance to the development zone. High quality development requires that the toner cloud be in a state which will enable it to be captured by fine details of the latent electrostatic image, the field lines of which are very local to the imaging surface. Toner transporting at too high a velocity or too close to the transport grid will not be developed to the image. The way we accomplish high quality development for mechanical donor roll powder cloud systems is to apply an AC field between the donor and the photoreceptor backplane to move the toner cloud closer to the image (donor AC).
However, noting the issues above the achievement of high reliability and simple, economic manufacturability of the system continue to present problems.